On Friday, I got a model which was set up in StormNET and as the model is run in SSA, got different output results for the hydraulic section, in comparison to the StormNET output report, even though the model was not changed and all the modeling parameters were same. First and foremost thing which came to my mind was the change in the EPA SWMM engine has caused the difference in the results.

Latest SSA version is using EPA SWMM version 5.0.021 in compare to the SNET using EPA SWMM version 5.0.014. However, I still had questions in my mind that what could be the other factors which contributed significantly for the different results for the links and the nodes in the two output reports. Though, SNET was not installed on my system. I took the help of Matt Anderson to compare the EPA SWMM 5.0.014 results with SSA. In the discussion came to know that apart from the change in EPA SWMM engine the culvert equation and the storage node floods (HGL above the node storage curve) contributed to the difference in the output results for SSA and SNET. I would like to share the data presented by Matt Anderson related to the corrections to the EPA SWMM engine between 5.0.014 (SNET) and 5.0.021 (SSA 2012).

A report error for the overflow rate into the ponded volume for a node that floods under dynamic wave flow routing was corrected 5.0.16

When the water level at a storage node exceeds the highest level supplied in its Storage Curve, an extrapolated surface area from the curve is now used only if the curve is sloping outward (i.e., surface area is increasing with depth at the top of the curve). If instead it's slope is inward then the last surface area entry in the curve is used.

When the ponding option is turned on, nodes that can pond are no longer always treated like storage nodes that never surcharge. Now they are only treated this way after ponding occurs. Otherwise they behave like a normal node.

The small tolerance used to decide when a storage node was full or not has been removed since for very small time steps it could cause a currently full storage unit to remain full even if there was some small net outflow from it.

5.0.17

The Ponding routine for dynamic wave flow routine was once again modified, this time to account for the special case where a node transitions between surcharged and ponded conditions within a single time step.

5.0.18

Double counting the final stored volume when finding the nodes with the highest mass balance errors has been eliminated.

5.0.19

Storage node losses from evaporation and infiltration are now computed directly within the flow routing routines to produce better conservation of mass.

The maximum trials used when evaluating the flow and head equations at a given time period for dynamic wave routing was increased from 4 to 8.

The ponding calculation for dynamic wave flow routing was changed again to obtain better continuity results. The depth in a surcharged node that can pond is not allowed to rise higher than just beyond full depth at any single time step. After that, it's change in depth is determined by the node's ponded area. Similarly, the depth of a ponded node is not allowed to drop more than just below full depth in any single time step.

The Villemonte correction for downstream submergence is now also used for partly filled orifices (instead of just for weirs).

.A missing term in the equation used to check for submerged inlet control for Culverts conduits was fixed.

The last item is probably the most important item to the model in question since the storage node's primary outlet's is a culvert

In the model main difference in the pond's storage appears to be that StormNET's volume is not releasing as quickly as SSA. In the Time Series Plot for Pond volume shows the volume increases for SNET, but decreases for SSA. A culvert coefficient was missing in the SNET compare to SSA, which made the rise in the pond volume in compare to SSA pond volume.

Time Series plot for the total inflow and the volume storage for the pond and outflow from the culvert and the weir in SNET.

Time Series plot for the total inflow and the volume storage for the pond and outflow from the culvert and the weir in SSA 2012.

The missing Culvert Parameter in the SNET version using EPA SWMM version 5.0.014 is evident in the two graphs.

The Time Series Plot for the Culver in SNET.

In the model in SSA, there is more weir and culvert flow in compare to SNET and less storage due to culvert coefficient. This clearly states the reasons for the difference in the output results of the same model for the nodes and links in SSA and SNET.

Hope you like this and stay tuned for more.